Macromolecular synthesis during plant embryogeny. Cellular rates of RNA synthesis in diploid and polytene cells in bean embryos

Using Giant Scarlet Runner Bean (Phaseolus coccineus) Embryos to Dissect the Early Events in Plant Embryogenesis

The giant embryo of the scarlet runner bean (Phaseolus coccineus) has been used historically to investigate the molecular and developmental processes that control the early events of plant embryo development. In more recent years, our laboratory has been using scarlet runner bean embryos to uncover the genes and regulatory events that control embryo proper and suspensor region differentiation shortly after fertilization. In this chapter we describe methods that we have developed to isolate scarlet runner bean embryos at the globular stage of development, and capture embryo proper and suspensor regions by either hand dissection or laser capture microdissection (LCM) for use in downstream genomic analysis. These methods are also applicable for use in investigating the early events of common bean (Phaseolus vulgaris) embryo development, a close relative of scarlet runner bean, which also has a giant embryo in addition to a sequenced genome.

A shared cis-regulatory module activates transcription in the suspensor of plant embryos

The mechanisms controlling the transcription of gene sets in specific regions of a plant embryo shortly after fertilization remain unknown. Previously, we showed that G564 mRNA, encoding a protein of unknown function, accumulates to high levels in the giant suspensor of both Scarlet Runner Bean (SRB) and Common Bean embryos, and a cis-regulatory module containing three unique DNA sequences, designated as the 10-bp, Region 2, and Fifth motifs, is required for G564 suspensor-specific transcription [Henry KF, et al. (2015) Plant Mol Biol 88:207-217; Kawashima T, et al. (2009) Proc Natl Acad Sci USA 106:3627-3632]. We tested the hypothesis that these motifs are also required for transcription of the SRB GA 20-oxidase gene, which encodes a gibberellic acid hormone biosynthesis enzyme and is coexpressed with G564 at a high level in giant bean suspensors. We used deletion and gain-of-function experiments in transgenic tobacco embryos to show that two GA 20-oxidase DNA regions are required for suspensor-specific transcription, one in the 5' UTR (+119 to +205) and another in the 5' upstream region (-341 to -316). Mutagenesis of sequences in these two regions determined that the cis-regulatory motifs required for G564 suspensor transcription are also required for GA 20-oxidase transcription within the suspensor, although the motif arrangement differs. Our results demonstrate the flexibility of motif positioning within a cis-regulatory module that activates gene transcription within giant bean suspensors and suggest that G564 and GA 20-oxidase comprise part of a suspensor gene regulatory network.

Using giant scarlet runner bean embryos to uncover regulatory networks controlling suspensor gene activity

One of the major unsolved issues in plant development is understanding the regulatory networks that control the differential gene activity that is required for the specification and development of the two major embryonic regions, the embryo proper and suspensor. Historically, the giant embryo of scarlet runner bean (SRB), Phaseolus coccineus, has been used as a model system to investigate the physiological events that occur early in embryogenesis-focusing on the question of what role the suspensor region plays. A major feature distinguishing SRB embryos from those of other plants is a highly enlarged suspensor containing at least 200 cells that synthesize growth regulators required for subsequent embryonic development. Recent studies have exploited the giant size of the SRB embryo to micro-dissect the embryo proper and suspensor regions in order to use genomics-based approaches to identify regulatory genes that may be involved in controlling suspensor and embryo proper differentiation, as well as the cellular processes that may be unique to each embryonic region. Here we review the current genomics resources that make SRB embryos a compelling model system for studying the early events required to program embryo development.

Regional localization of suspensor mRNAs during early embryo development

We investigated gene activity within the giant embryos of the scarlet runner bean (Phaseolus coccineus) to gain understanding of the processes by which the apical and basal cells become specified to follow different developmental pathways after division of the zygote. We identified two mRNAs, designated G564 and C541, that accumulate specifically within the suspensor of globular-stage embryos. G564 mRNA accumulates uniformly throughout the suspensor, whereas C541 mRNA accumulates to a higher level within the large basal cells of the suspensor that anchor the embryo to the surrounding seed tissue. Both G564 and C541 mRNAs begin to accumulate shortly after fertilization and are present within the two basal cells of embryos at the four-cell stage. In contrast, at the same stage, these mRNAs are not detectable within the two descendants of the apical cell. Nor are they detectable within cells of the embryo sac before fertilization, including the egg cell. We used a G564/beta-glucuronidase reporter gene to show that the G564 promoter is activated specifically within the basal region and suspensor of preglobular tobacco embryos. Analysis of the G564 promoter identified a sequence domain required for transcription within the suspensor that contains several copies of a conserved motif. These results show that derivatives of the apical and basal cells transcribe different genes as early as the four-cell stage of embryo development and suggest that the apical and basal cells are specified at the molecular level after division of the zygote.

Regional localization of suspensor mRNAs during early embryo development

We investigated gene activity within the giant embryos of the scarlet runner bean (Phaseolus coccineus) to gain understanding of the processes by which the apical and basal cells become specified to follow different developmental pathways after division of the zygote. We identified two mRNAs, designated G564 and C541, that accumulate specifically within the suspensor of globular-stage embryos. G564 mRNA accumulates uniformly throughout the suspensor, whereas C541 mRNA accumulates to a higher level within the large basal cells of the suspensor that anchor the embryo to the surrounding seed tissue. Both G564 and C541 mRNAs begin to accumulate shortly after fertilization and are present within the two basal cells of embryos at the four-cell stage. In contrast, at the same stage, these mRNAs are not detectable within the two descendants of the apical cell. Nor are they detectable within cells of the embryo sac before fertilization, including the egg cell. We used a G564/beta-glucuronidase reporter gene to show that the G564 promoter is activated specifically within the basal region and suspensor of preglobular tobacco embryos. Analysis of the G564 promoter identified a sequence domain required for transcription within the suspensor that contains several copies of a conserved motif. These results show that derivatives of the apical and basal cells transcribe different genes as early as the four-cell stage of embryo development and suggest that the apical and basal cells are specified at the molecular level after division of the zygote.

Polytene chromosomes in mammalian cells

This article deals with the structural and functional organization of polytene chromosomes in mammals. Based on cytophotometric, autoradiographic, and electron microscopic data, the authors put forward a concept of nonclassic polytene chromosomes, with special reference to polytene chromosomes in the mammalian placenta. In cells with nonclassic polytene chromosomes, two phases of the polytene nucleus cycle are described, such as the endointerphase (S phase) and endoprophase (G phase). The authors generalize that the main feature of nonclassic polytene chromosomes is that forces binding the sister chromatids are much weaker than in the Diptera classic polytene chromosomes. This concept is confirmed by comparative studies of human, mink, and fox polytene chromosomes. The final step of the trophoblast giant cell differentiation is characterized by a transition from polyteny to polyploidy, with subsequent fragmentation of the highly polyploid nucleus into fragments of low ploidy. Similarities and dissimilarities of pathways of formation and rearrangement of nonclassic polytene chromosomes in mammals, insects, plants, and protozoans are compared. The authors discuss the significance of polyteny as one of the intrinsic conditions for performance of the fixed genetic program of trophoblast giant cell development, a program that provides for the possibility of a long coexistence between maternal and fetal allogenic organisms during pregnancy.

Ultrastructure of decidualization in the pseudopregnant rat

The ultrastructure of the endometrial stroma in rats bearing deciduomata was examined in detail on day 9 (day of vaginal cornification = day 0) of pseudopregnancy, together with the development of deciduomata from day 4 to day 8. Five major regions were recognizable on day 9. 1) In the basal zone, which contributes to endometrial regeneration following decidualization, stromal cells remained fibrocyte-like and were separated by wide bands of collagen fibrils. Capillaries were fenestrated and large. 2) The capsule surrounding the antimesometrial (region of the) deciduoma was composed of flattened cells, showing mitosis, which appeared to provide a source of appositional growth of the deciduoma. 3) The large, tightly packed, polyploid cells of the antimesometrial deciduoma showed morphological evidence of protein synthesis activity, an absence of smooth endoplasmic reticulum, and surface specializations suggestive of adhesion (adherens junctions), communication (gap junctions), and maintenance of an extensive surface area (lamellar processes). These cells came into very close apposition with capillary endothelial cells, which were seldom fenestrated, lacked any continuous basal lamina, and showed evidence of high metabolic activity. 4) The loosely packed mesometrial deciduoma contained a structurally supportive, "fixed" population of spiny cells and a "free" population of granulated cells and their putative precursors, together with a meshwork of large, sinusoidal capillaries whose endothelium was generally supported by a basal lamina. 5) The peripheral parts of the glycogenic area, which was structurally similar to the mesometrial deciduoma, contained many grossly enlarged intercellular spaces of undetermined function. The stromal cells at day 4, which give rise to all non-vascular elements except the granulated cells mentioned above, were structurally simple and fibrocyte-like. Only two distinct regions, subepithelial and deep, were recognizable. Capillaries at day 4 generally possessed a continuous endothelium, with a complete basal lamina and many pericytes. Between days 5 and 8, the stromal cells showed divergent forms of differentiation according to their position in the endometrium. Ultrastructural differentiation of the vasculature also showed divergence according to position, leading to the various types of capillaries seen at day 9. Regional variation, and high levels of structural organization, characterize the "programmed" decidual response in the pseudopregnant rat.

Suspensor, gibberellin and in vitro development of Phaseolus coccineus embryos

Embryos of Phaseolus coccineus in different stages of development (from 0.5 to 5 mm in length) were grown in vitro. Both intact embryos (with suspensor) and embryos deprived of suspensor were studied. It was found that removal of the suspensor has no effect on the development of embryos which have reached a length of 5 mm. With younger embryos, removal of the suspensor reduces embryo development, the negative effect being the greater the younger the embryo. It was shown that gibberellic acid (GA3) concentrations of 10(-8) to 10(-6)M can replace the suspensor in heart-shaped and early cotyledonary embryos (0.5 to 1.5 mm in length), whereas they reduce the development of suspensor-deprived embryos of later stages (embryos 2 to 3 mm in length) as compared with intact embryos of similar size grown on hormone-free medium. GA3 concentrations of 10(-5) and 10(-4)M are generally inhibitory and may stimulate callus formation in some embryos. The present data and those of Alpi et al. (1975) concur in ascribing a major role to gibberellins in characterizing the physiological function of the suspensor in early embryogenesis in Phaseolus coccineus.

Growth regulator levels in embryo and suspensor of Phaseolus coccineus at two stages of development

Gibberellins and auxins were extracted from embryos and suspensors of Phaseolus coccineus L. at two stages of development: A) heart-shaped embryo and B) cotyledonary embryo with suspensor in the initial stage of degeneration. The time interval between the two stages was 5-6 days.In both embryos and suspensors, gibberellin (GA)-like activity was found in three fractions: F-1 (ethyl acetate fraction at pH 8.0), F-2 (free GAs) and F-3 (bound GAs). At stage A, the total GA activity in the suspensor was about 30 times greater than in the embryo and the bound GAs contributed by about 90% to the total GA content. A dramatic decrease in level of bound GA-like substances was found in suspensors at stage B, when the level of total GAs in the embryo had increased to 10 times that at stage A. This might suggest a transport of GAs from the suspensor to the embryo. In both embryo and suspensor, qualitative changes in GAs with shift in activity of the fractions tested occurred at the two developmental stages.The methanolic extracts of stage A suspensors showed two inhibitors, one much more active than the other, and two large peaks of growth promoting activity at Rf 0.4-0.7; in stage A embryos, the general activity of the extracts was lower and the promoting effect was spread over Rf 0.3-0.9.The present results seem to support the view that the suspensor plays a role in embryogenesis by acting as a site of synthesis of growth regulators needed by the embryo.